A fuel cell is a system for producing electric power. In a fuel cell, chemical reaction energy between oxygen and hydrogen contained in hydrocarbon-group materials (e.g., methanol, natural gas) is directly converted into electric energy. Such a fuel cell is characterized by the production of electric energy and thermal energy as a by-product of an electrochemical reaction occurring without combustion.
Depending on the type of electrolyte used in a fuel cell, the fuel cell may be classified into one of many different types of fuel cells, for example, phosphate fuel cells, molten carbonate fuel cells, solid oxide fuel cells, and polymer electrolyte or alkali fuel cells. Although each of these different types of fuel cells operate using the same principles, they differ in the type of fuel, catalyst, and electrolyte used, as well as in drive temperature.
A polymer electrolyte membrane fuel cell (PEMFC) has been developed recently. Compared to other fuel cells, the PEMFC has excellent output characteristics, a low operating temperature, and fast starting and response characteristics. The PEMFC may be used for vehicles, in the home and in buildings, and for the power source in electronic devices. The PEMFC, therefore, has a wide range of applications.
The basic components of the PEMFC are a stack, a fuel tank, and a fuel pump. The stack forms a main body of the fuel cell. The fuel pump supplies fuel reserved in the fuel tank to the stack. A reformer may also be used to reform the fuel to create relatively pure hydrogen gas and to supply the hydrogen gas to the stack.
In the PEMFC, the fuel pump operates to send the fuel from the fuel tank to the reformer. The fuel is reformed in the reformer to generate hydrogen gas, and the hydrogen gas is chemically reacted with oxygen in the stack to generate electric energy.
Fuel cells using Direct Methanol Fuel Cell (herein referred to as “DMFC”) supply liquid methanol fuel containing hydrogen directly to the stack and therefore may not include a reformer. This lack of a reformer is a difference between the PEMFC and DMFC.
FIG. 8 is a partial cross section of a stack used in a fuel cell system according to the prior art where MEAs are assembled with separators.
With reference to FIG. 8, some embodiments of a fuel cell may include a stack for substantially generating electricity. The stack may be composed of a structure of stacked unit cells. The stacked unit cells may contain a few unit cells, or ten or more unit cells having MEAs 51 and separators 53.
The MEAs 51 have an electrolyte membrane and an anode electrode and a cathode electrode mounted on opposite surfaces thereof. The separators 53 have passages 55, 57 through which the hydrogen gas and/or air needed for the oxidation/reduction reaction of the MEAs 51 is supplied to the anode electrode and the cathode electrode.
That is, the hydrogen gas is supplied to the anode electrode and the air is supplied to the cathode electrode through passages 55 and 57, respectively, of the separator 53. In this process, the hydrogen gas oxidizes at the anode electrode and the oxygen reduces at the cathode electrode. The flow of electrons generated during this operation creates a current. In addition, water and heat are generated by the electrochemical reactions.
In more detail, each separator 53 includes plurality of ribs 59 closely faced against the adjacent surfaces of MEAs which define the passages 55, 57 for supplying the hydrogen gas and air needed. Substantially, the passages interpose between each of the ribs 59.
Generally, where separators are positioned on both sides of MEAs 51, the passages 55, 57 for supplying each of the hydrogen gas and air needed are orthogonal to each other. Thus, in the cross-section illustrated in FIG. 8, a single passage 55 for supplying the hydrogen gas is illustrated while a plurality of passages 57 for supplying air are illustrated.
In the fuel cell system described above, the structure of a stack should enhance the diffusing performance in the stack while maintaining the pressure of the fuel during diffusion in order to enhance the efficiency of fuel cell. Here, one important condition for designing the structure of a stack is the size of the passages 55, 57. That is, in the separator 53, the size of passages plays an important role in diffusing hydrogen gas and air to diffusing layers thereof from the active area of MEA 51, and also for handling the contact resistance of current generated in MEA 51.